The invention relates to a joining method for welding metal members together by a friction stirring.
There is known a friction stir welding (FSW) for joining metal members together. The friction stir welding is a method in which a rotation tool is rotatingly traversed along the butt portion between metal members, to plastically fluidize a metal or metals along the butt portion with the frictional heat produced between the rotation tool and the metal members, thereby solid-phase joining the metal members together.
For example, Patent Document 1 (Japanese Patent Application Laid-open No. 2008-290092) discloses a technique that the friction stir welding is applied to the butt portion between metal members by use of a rotation tool including a shoulder section and a stirring pin protruding from the lower end surface of the shoulder section.
For example, Patent Document 2 (Japanese Patent Application Laid-open No. 2002-79383) discloses a technique that plate-shaped metal members are stacked on each other before a rotation tool is inserted from the surface of an upper-side metal member to apply a friction stirring to the lap portion.
In each friction stir welding according to the Patent documents 1 and 2, the friction stirring is performed by pushing the lower end surface of the shoulder section of the rotation tool into the surface of the metal member by a few millimeters.
When metal members each including an inclined surface or a curved surface are butted against each other, the resulting butt portion has various heights. When plate-shaped metal members each including an inclined surface or a curved surface are stacked on each other, the resulting lap portion has various heights. In these cases, if a conventional rotation tool is used to perform a friction stir welding, the shoulder section of the rotation tool contacts with the inclined surface or the like, which makes it difficult for the rotation tool to travel. Further, when the friction stir welding is applied to the butt portion or the lap portion with the inclined surface or the like, it is difficult for a stirring pin to be inserted into the butt portion or the lap portion at a deep position, thereby occasionally causing a joint defect.
On the other hand, there is another case in which the friction stir welding is applied to a lap portion which is formed by stacking a first metal member having a front surface with various heights on a second metal member with a constant plate thickness, with the first metal member's back surface in contact with the second metal member's front surface, and the friction stir welding is applied to the lap portion thus formed. In this case also, traveling of the rotation tool is obstructed by the changes in height of the front surface of the first metal member, a surface on the side from which the rotation tool is inserted, which causes a joint defect in the conventional rotation tool.